President Trump said on Tuesday that he planned to order the military to guard parts of the southern border until he can build a wall and tighten immigration restrictions, proposing a remarkable escalation of his efforts to crack down on migrants entering the country illegally.

Mr. Trump, who has been stewing publicly for days about what he characterizes as lax immigration laws and the potential for an influx of Central American migrants to stream into the United States, said he was consulting with Jim Mattis, the secretary of defense, about resorting to military deployments.

“We have very bad laws for our border, and we are going to be doing some things — I’ve been speaking with General Mattis — we’re going to be doing things militarily,” Mr. Trump said at the White House, seated beside the defense secretary at a meeting with visiting leaders of Baltic nations. “Until we can have a wall and proper security, we’re going to be guarding our border with the military. That’s a big step. We really haven’t done that before, or certainly not very much before.”

It was not immediately clear what Mr. Trump meant by the remarks, or what the rationale would be for deploying United States troops to patrol or even seal the border at a time when the numbers of people being apprehended crossing illegally are down to their lowest level since 1971.

The active-duty military is generally barred by law from carrying out domestic law enforcement functions, such as apprehending people at the border. But previous presidents have deployed National Guard troops to act in support roles on the border with Mexico — former President Barack Obama sent 1,200 in 2010 and former President George W. Bush dispatched 6,000 in 2006.Governors of border states have done the same when faced with large inflows.

Last February, he called his immigration crackdown “a military operation,” prompting Rex W. Tillerson, then the secretary of state, and John F. Kelly, then the homeland security secretary, who were visiting Mexico at the time, to push back vigorously. They told their Mexican counterparts and reporters that the American president did not, in fact, plan to use the military to hunt down and deport undocumented immigrants. The White House later insisted that Mr. Trump had meant the word “military” only as an adjective.

On Tuesday, though, the president appeared convinced that American troops were needed.

“We are preparing for the military to secure our border between Mexico and the United States,” he said during a news conference with the Baltic leaders, adding that he would be meeting with Mr. Mattis and other officials on the matter later in the day. “I think it’s something we have to do.”

Mr. Trump’s comments on Tuesday came after he kicked off his third consecutive day of tweeting about America’s “weak” border laws and called on Congress to act on legislation to toughen immigration laws. The push comes as Mr. Trump has complained with increasing urgency about a large group of migrants from Honduras that has been traveling through Mexico.

The caravan has been a popular topic on Fox News — the president’s favorite news network — and his aides have argued that weak immigration policies are luring the migrants from Central America to the United States.